The Writer's Ideal Reader
Whenever a writer sits down to produce a work, it is not a solitary activity. While the writer may be alone in the room and silent other than the scratching of a pen or the tapping of a keyboard, the fact is that the person writing is communicating. Very rarely will a writer, or anyone else, communicate without having a specific person in mind, in other words, the ideal reader. This ideal reader captures all of the characteristics that the writer expects in his or her audience. While not every reader is ideal, the fact is that the writer makes a number of underlying assumptions about the people who will be reading his or her work.
Past Knowledge
Perhaps the greatest assumption to be made by a writer is that the reader will have the necessary knowledge to fully appreciate the work at hand. After all, few second grade students tote around a copy of an advanced astrophysics text because they lack the past knowledge to fully understand it. This is also true for many other works. When writing a business letter, the businessperson may make reference to past deals or conversations, but assumes that the recipient understands what is being talked about. To a lesser degree, this is also true with series of books. An author may revisit some established details in a sequel to aide any new readers, but in many ways the characters and setting are already established by earlier works. The author expects the reader to know that.
Information about the Author
Speaking of the author, there are assumptions that are necessary to make about that individual as well. For example, when someone reads a non-fiction book, they assume that the author knows what they are talking about. After all, it would be a shame to trudge through that text on advanced astrophysics to find out that the author is actually that second grade student who was carrying it around.
Additionally, past experiences matter here as well. The reader can be expected to understand that an author has an interest in the field. For example, in that aforementioned business letter, we assume that the author has the connections necessary to presume to write to the subject.